1. Field of the Invention
This invention relates to a device for forming a deposited film to be used for formation of a functional film, particularly a functional deposited film which is useful for an electronic device such as a semiconductor device, photosensitive device for electrophotography, optical input sensor for optical image inputting device, image pick-up device, photovoltaic device, etc.
2. Related Background Art
In the prior art, for formation of amorphous or polycrystalline functional films such as semi-conductor films, insulating films, photoconductive films, magnetic films, metal films, etc., suitable film forming methods have been individually employed from the standpoint of desired physical characteristics, uses, etc.
For example, for formation of silicon deposited films such as of non-single crystalline silicon including amorphous and polycrystalline silicon which are optionally compensated for lone pair electrons with a compensating agent such as hydrogen atoms (H) or halogen atoms (X) , etc., (hereinafter abbreviated as "NON-Si (H,X)" particularly "A-Si (H,X)" when indicating amorphous silicon and "poly-Si (H,X)" when indicating polycrystalline silicon) (the so called microcrystalline silicon is included within the category of A-Si (H,X) as a matter of course), there have been used as an attempt the vacuum vapor deposition method, the plasma CVD method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc. Generally, the plasma CVD method has been widely used and industrialized. These methods can also be used for formation of other deposited films.
However, the reaction process in formation of a silicon deposited film according to the plasma CVD method which has been generalized in the prior art is considerably complicated as compared with the CVD method of the prior art, and its reaction mechanism involves not a few ambiguous points. Also, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rates and flow rate ratio of introduced gases, pressure during film formation, high frequency power, electrode structure, structure of reaction vessel, evacuation rate, plasma generating system, etc.). On account of dependence on such a large number of parameters, plasma may sometimes become an unstable state, whereby marked deleterious influences were exerted frequently on a deposited film formed. Besides, parameters specific to individual devices must be selected for each device and therefore under the present situation it is actually difficult to standardize the production conditions.
On the other hand, for silicon deposited films to exhibit sufficiently satisfactory electrical or optical characteristics for respective uses, it is now accepted that the best way to form them is according to the plasma CVD method.
However, depending on the application use of silicon deposited films, bulk production with reproducibility may be required with full satisfaction in terms of enlargement of area, uniformity of film thickness as well as uniformity of film quality, and therefore in formation of such silicon deposited films according to the plasma CVD method, enormous installation investment will be required for a bulk production device and also control items for bulk production will be complicated with a narrow tolerance limit for control and a delicate operating condition of a device. These are pointed out as problems to be improved in future.
Also, in the case of the plasma CVD method, since plasma is directly generated by high frequency or microwave, etc., in a film forming space in which a substrate for film formation is arranged, electrons or a number of ion species generated may cause damage to a film in the film forming process to cause lowering in film quality or non-uniformization of film quality.
For the improvement of this point, the indirect plasma CVD method was proposed.
The indirect plasma CVD method was developed to use selectively effective chemical species for film formation by forming plasma by microwave, etc., at an upstream position apart from a film forming space and transporting said plasma to the film forming space.
However, even in the indirect plasma CVD method, transport of plasma is essentially required and therefore the chemical species effective for film formation must have long life, whereby the gas species which can be employed are spontaneously limited, thus failing to give various deposited films. Also, enormous energy is required for generation of plasma, and generation of the chemical species effective for film formation and their amounts cannot be essentially placed under simple control. Thus, various problems remain to be solved.
As contrasted to, the plasma CVD method, the optical CVD method is advantageous in that no ion species or electrons are generated which give damages to the film quality during film formation. However, there are problems such that light source does not include so much kinds, that the wavelength of light source tends to be toward UV-ray side, that a large scale light source and its power source are required in the case of industrialization, that a window for permitting light from a light source to be introduced into a film forming space is coated with a film during film formation to result in lowering in dose during film formation, which may further lead to shut-down of the light from the light source into the film forming space.
There is proposed recently a new method for forming a deposited film quite different in film forming process from the above-mentioned methods. FIG. 1 illustrates an embodiment of the apparatus realizing such a method for forming a deposited film as a schematic drawing.
The deposited film forming device shown in FIG. 1 is broadly divided into a main body, an evacuation system and a gas feeding system.
In the main body (vacuum chamber), a reaction space and a film forming space are provided.
101-105 are respectively bombs filled with gases to be used during film formation, 101a-105a are respectively gas feeding pipes, 101b-105b are respectively mass flow controllers for controlling the flow rates of gases from the respective bombs, 101c-105c are respectively gas pressure gauges, 101d-105d and 101e--105e are respectively valves, and 101f-105f are respectively pressure gauges indicating the pressures in the corresponding gas bombs.
120 is a vacuum chamber equipped at the upper portion with a means for gas introduction, having a structure for formation of a reaction space downstream of the gas introducing means, and also having a structure for formation of a film forming space in which a substrate holder 112 is provided so that a substrate 118 may be placed as opposed to the gas 10 introducing port of said means. The means for gas introduction has a double concentric arrangement structure, having from the innerside a first gas introducing pipe 109 for introducing gases from the gas bombs 101, 102, and a second gas introducing pipe 110 for introducing gases from the gas bombs 103-105. 111 is the tip end portion of the gas introducing pipes 109 and 110 and constitutes the gas introducing port. The dimension and arrangement of the gas introducing pipes 109 and 110 at the gas introducing port 111 are same as those at the upflow side, that is, they are in a straight double concentric structure. Further, the ends of the gas introducing pipes 109 and 110 are arranged evenly within a single plane. In this case, since the gas introducing port to the reaction space has the above-explained structure, gases from the gas introducing pipes 109 and 110 are individually introduced into the reaction space and then mixed therein.
Gases from the gas bombs are fed to the respective introducing pipes through the gas feeding pipelines 123 and 124, respectively.
The respective gas introducing pipes, the respective gas feeding pipelines and the vacuum chamber 120 are adapted to be evacuated to vacuum through the main vacuum valve 119 by means of an evacuating device not shown.
A substrate 118 is suitably placed at a desired distance from the positions of the respective gas introducing pipes by moving vertically the substrate holder 112.
113 is a heater for heating a substrate which is provided in order to heat a substrate to an appropriate temperature during film formation, or preheating a substrate 118 before film formation, or further to anneal a film after film formation.
The substrate heating heater 113 is supplied with power through a conductive wire 114 from a power source 115.
116 is a thermocouple for measuring the temperature of a substrate (Ts) and is electrically connected to a temperature display device 117.
An example of film forming process with the use of the device for formation of a deposited film shown in FIG. 1 is described below.
At first, after completing the predetermined gas supply procedure, SiH.sub.4 gas filled in the bomb 101 and F.sub.2 gas diluted to 5% with He gas (referred to F.sub.2 (5)/He gas) are introduced to the reaction space at the gas blowing port 111 through the gas introducing pipes 109 and 110 respectively. SiH.sub.4 gas and F.sub.2 (5)/He gas introduced into the reaction space are therein mixed with each other and chemical reaction is caused by the oxidization action of F.sub.2 gas. A substrate 118 is then exposed to the atmosphere where the chemical reaction is in progress and an Si:H:F film is thereby formed on the substrate 118.
As above, a deposited film forming process using the device for forming a deposited film shown in FIG. 1 has the advantage that only mixing in the reaction space of gases suitably selected and used as desired enables film formation. The device shown in FIG. 1 is not so complicated and expensive as devices used in e.g. the PCVD process. Further, the film suffers from no ion damage or particle damage and thus a film of high quality can be formed.
The deposited film forming process described above is very simple while the gas mixing technique in the reaction space is an important parameter which decides the quality of a film formed.
As described above, in formation of silicon deposited film, points to be solved still remain, and it has been earnestly desired to develop a method for forming a deposited film which is capable of bulk production with conservation of energy by the use of a device of low cost, while maintaining the practically utilizable characteristics and the uniformity. These are in common with the cases of other functional films such as silicon nitride films, silicon carbide films, silicon oxide films for the similar problems which should be solved respectively.